At present, machine tools utilize measuring systems/devices which take positional measurements of a movable machine tool part along the part's plane of movement. A typical direct position measuring system of the type described above can be found in German Patent No. 12 14 307. The patent describes a measuring system having a measurement scale which is designed as purely incremental, for instance, as a glass scale with divisions or as an inductive linear scale, in which, the voltages generated by the movement of the machine tool part and taken off are ultimately reconverted into pulses that are then counted. The above-mentioned patent also teaches that several periodically graduated analog measuring systems can be provided simultaneously in order to cover a large measuring range.
One disadvantage of a direct position measuring system as described is the relatively low resolution of the measurements. In any such incremental position measuring system, the range of measuring uncertainty is plus or minus one (.+-.1) increment. Secondly, this range of uncertainty contributes to a hysteresis effect in the position control feedback loop of the drive control system whereby the position feedback signal can undergo amplitude oscillations of a magnitude of .+-.1 increment. This, in turn, causes the movable part of the machine tool to oscillate during movement and stopping.
It is therefore an object of the present invention to develop a position measuring apparatus whose resolution is at least two to three times greater than the desired measuring accuracy in order to keep the reactions on the workpiece small.
It is also an object of the present invention to develop a position measurement apparatus whose measuring accuracy can be increased substantially by relatively simple means.